1. It had snowed that night and there were no footprints.
2. The ransom note was written on a notepad found in the house.
3. The killer molested/murdered the little girl in the house. How many pedophiles do that? They always try to leave with the child so no one will catch them.
4. The house was huge and had a very complicated lay out. The "stranger" entered, found their way (in the dark) to the little girl's room and was never heard.